when; while 当……时候while能用when代替; 但是when却不一定能用while代替.while+从句, 动作一定会延续when+延续性动词/瞬间动词;
由when引导的时间状语从句,主句用过去进行时,从句应用一般过去时；如果从句和主句的动作同时发生,两句都用过去进行时的时候,多用while引导,如：
a. When the teacher came in, we were talking.
当此句改变主从句的位置时,则为：
While we were talking, the teacher came in.
b. They were singing while we were dancing.